Rotorcraft Technology: Development and Prospects

Helicopters boast greater versatility than fixed-wing aircraft, in their range of operating sites and in their ability to hover. They however fly substantially slower with most designs limited to around 315kph (170kt). Efforts to produce faster rotorcraft over the years resulted in heavier, more complex and expensive and less efficient aircraft. However the advance of technology now appears to offer better prospects.

Compound Helicopter
Conceptually, increasing forward thrust provides the simplest way to increase speed, as in compound helicopters, with their supplementary cruise engine(s).

Sikorsky conceived its X2 technology demonstrator to significantly improve helicopter speeds, aiming for 460kph (250kt). Configured with a counter-rotating coaxial main rotor, pusher propeller and fly-by-wire (FBW) flight control system and a LHTEC T800-801 turboshaft engine, it first flew in August 2008 and the company plans to attain its speed target by the end of this year.

A production version of the X2 though would require progress in stability, controllability, weight, power and reliability and would cost significantly more than a conventional helicopter.

In its X-49A program Piasecki pursues high speed through the addition of a vectored-thrust ducted propeller (VTDP) to a conventional helicopter design, the Sikorsky H-60, powered by two General Electric T700-GE-401C turboshafts. It first flew in June 2007.

Eurocopter studies too could encompass a high speed hybrid helicopter demonstrator, the X3, which could use the Rolls-Royce RTM322 engine.

Russian compound helicopter concepts include the Mil Mi-X1 with a thrust vectoring tail propeller and Kamov Ka-92 with coaxial main rotors and a tail propeller.

Engine retrofits on existing helicopters could improve speed and performance, Piasecki suggesting the addition of two thrusters on each fuselage side of a Boeing CH-47 Chinook tandem rotor helicopter.
 
Tiltrotor
Tiltrotor aircraft can achieve even higher speeds, with the Bell/Agusta BA609, now in flight test, designed to cruise at 510 kph (275kt) and attaining 570kph (310kt) in test.

Also, although more complex than conventional rotorcraft, tiltrotor technology benefits from the military’s operational deployment of such an aircraft, the US forces’ Bell Boeing V-22.

However, vertical flight requires rotors operate at high speeds, which makes them inefficient in cruise, also when compared to fixed-wing aircraft.

Using optimum speed tiltrotor (OSTR) technology US-based Karem Aircraft attempts to optimise the rotor speed for vertical and forward flight. Its TR53 AeroTrain design, comparable in size to a Boeing 737-600 airliner, features a tiltrotor on each wing.

According to Karem Aircraft the TR53’s large rotor diameter of 16 metres in effect allows a much higher bypass ratio compared to a turbofan like the 1.55m-diameter CFM56 on the B737 and therefore more efficient flight; although the TR53 would fly slower than a B737 (Mach 06. compared to 0.8).

In vertical flight it will use much more fuel, but according to Karem it will spend only a small proportion of its flight time in that mode and its efficiency in cruise more than balances its inefficiency in vertical flight, making it overall more efficient than fixed-wing aircraft.

The TR53 would initially use the Rolls-Royce AE1107C as used on the V-22 with later an advanced engine designed for the optimum speed mechanism.

The US NASA Subsonic Rotary Wing program, part of its Fundamental Aeronautics studies to overcome challenges to air transport in the US, will consider performance, efficiency and noise, among other factors via-a-vis promising rotorcraft technologies.

Research areas include propulsion; flight dynamics and control; aeromechanics; acoustics; and materials and structures.

Technologies include tiltrotors and slowed-rotor compound helicopters and variable/multi-speed drive systems.

NASA research indicates great potential in large, fast civil tiltrotors, their rotor tip speeds approximately halving from hover to cruise: the organization’s Large Civil Tiltrotor 2nd Generation (LCTR2) design with 650 and 350fps hover and cruise tip speeds would carry 90 passengers with a 300kt cruise speed and 1250nm range.

A gyroplane (also known as autogyro or gyrocopter) obtains lift from a free-turning rotor and thrust from an engine-powered propeller. When moving forward, due to the propeller’s thrust, oncoming air rotates the rotor, producing lift. It will fly faster than a helicopter but since it needs an air flow to generate lift cannot hover. It will also fly slower than a fixed-wing aircraft.

A gyrodyne features a powered rotor for take-off/landing and hovering. Groen Brothers Aviation in the US envisage converting an existing production aircraft into a gyrodyne, for which only the tip-jets, rotor blades, rotor head and mast and flight control system require development, the company already modifying a Cessna Skymaster to the Hawk 4 gyroplane. It foresees 19-, 35-, 50- and 75- seat GyroLiners.

European Programs
While US programs focus on advancing vehicle technology, various pan-European programs examine alleviating the consequences of technology, including reducing noise and emissions.

Friendcopter, a European Union (EU) 6th Framework program that ended in August 2008, involved more than 30 European aircraft companies and research centres, including Eurocopter and AgustaWestland, and aimed to reduce helicopter acoustic footprint areas by up to 50per cent, fuel consumption and cabin noise and vibration, vehicle foci including engine inlets and outlets and distributed blade actuation.

Blade slap noise on approach would reduce by altering flight procedures, including glide path angle and speed; cabin noise by modifying the gearbox, reducing structure-transmitted noise between the gearbox and fuselage, and active structural control primarily on cabin panelling; and engine noise by treating inlets and outlets. Active blade control through distributed blade actuation would disperse blade tip vortices that cause blade slap noise and reduce the engine power needed.

Also within the 6th Framework, the Novel Innovative Competitive Effective Tilt Rotor Integrated Project (NICETRIP) aims to validate the tiltrotor concept based on the AgustaWestland Enhanced Rotorcraft Innovative Concept Architecture (ERICA) design, looking at vehicle configurations for long range and high speed.
The program would evaluate components on rigs and models in wind tunnels, consider the air traffic management of tiltrotors and their operations in the social and environmental contexts and outline the steps to a flying demonstrator. NICETRIP continues to 2011.

The 6th Framework Optimized Procedures and Techniques for IMprovement of Approach and Landing (OPTIMAL) program considered the integration of helicopters in the air traffic system.

Also EU-sponsored, the Clean Sky Joint Technology Initiative, a joint venture between public and private organizations, launched in early 2008 and to continue to 2015, includes the Green Rotorcraft Platform integrated technology demonstrator that seeks to reduce gaseous emissions through greater fuel efficiency and also to reduce noise output.

AgustaWestland and Eurocopter lead other members, including smaller corporate and research organizations. Clean Sky will progress from Friendcopter, OPTIMAL and NICETRIP.

Studies will include both helicopters and tiltrotors and encompass rotor blades, airframe drag, engine fit for low noise, turboshaft and diesel engine efficiency and emissions, electrical systems to reduce fuel use, ecodesign of the airframe and environment-friendly flight paths. Promising technologies will undergo further evaluation.

AgustaWestland leads studies into lowering external helicopter noise, including active blade control, aiming to halve it by 2020. Airframe drag studies would use the NH90 and ERICA as the helicopter and
tiltrotor models respectively, both designs involving AgustaWestland and Eurocopter.

The program will look at improving fuel efficiency and reducing emissions on a helicopter turboshaft engine. It also aims to lower specific fuel consumption by 30per cent and carbon dioxide and NOx emissions by 40 and 53 per cent respectively on a diesel-engined Eurocopter EC120 helicopter.

Later the EC120 could flight test a more innovative engine type. 2011 will see the choosing of technologies and configurations, 2012 ground and flight demonstrations and 2014 and 2015 flight test and evaluation.

In June at the Paris air show, Eurocopter revealed its Bluecopter technology demonstrator, intended to produce less emissions through the use of an exhaust gas cleaning system. According to Eurocopter the technology applies to small helicopters, with studies underway on those relevant to larger models.

Helicopter Systems
Sikorsky is studying hub-mounted active vibration suppressor (HMVS) technology, lighter and more aerodynamic than passive systems and also active blade control.

AgustaWestland continues development of its AW101 helicopter’s active control of structural response (ACSR), which uses computer-controlled forces to reduce vibration and noise and improve rotor efficiency.

AW101s could also use the British Experiemental Rotor Programme IV (BERP IV) rotor blades that improve lift and performance.

Following recent accidents, the US Federal Aviation Administration could rule that from 2011 emergency medical services (EMS) helicopters employ radar altimeters, terrain awareness and warning systems (TAWS) and cockpit voice and data recorders.

Honeywell modified its Primus Epic avionics suite, used on larger, fixed-wing aircraft, for the AgustaWestland AW139 helicopter, including for demanding search and rescue (SAR) and EMS missions.
Canada-based CMC Electronics plans to develop an integrated cockpit for helicopters.

The German Aerospace Centre (DLR) studies satellite navigation technology to enable precision helicopter approaches.

Honeywell evaluated technologies like enhanced ground proximity warning systems (EGPWS) and synthetic vision systems (SVS) on a Eurocopter A350 and AgustaWestland AW139 and also studies millimetre-wave radar for wire identification.

Following the evaluation of remote control of an un-piloted Little Bird helicopter, Boeing studies a control system to allow optionally manned aircraft, while Sikorsky foresees that its future helicopters will operate both piloted and un-piloted.
 
Current Helicopters
AgustaWestland’s current civil helicopters comprise the AW109, AW119, Grand, AW139 and AW101.

The company developed the Da Vinci variant of the Grand light twin with a modified electronic flight instrument system (EFIS) that includes an automatic flight control system and synthetic vision and terrain awareness warning system, radio management system, digital map, enhanced vision system (EVS) and night vision goggle (NVG) capability.

The forthcoming AgustaWestland XX9 will offer capabilities between the Grand and AW139.

The AW139 should receive certification for single-pilot instrument flight rules (IFR) and NVG operations in the USA and updates to the Primus Epic avionics.

Bell’s current civil range consists of the 206L4, 407 and 412. The newest type, the 429 IFR-capable light twin, which first flew in February 2007, experienced control system software problems that delayed certification to mid- 2009. It uses a dual digital three-axis autopilot with a stability control and augmentation system, X-shaped tail rotor and Pratt & Whitney Canada PW207D2 turboshaft engines and could accommodate eight passengers. Roles comprise corporate, EMS and utility.

Eurocopter offers the widest range of civil helicopters of any manufacturer – the EC120B, AS350B2/B3, EC130B4, AS355NP, EC135P2/T2i, EC145, AS365N3, EC155B1, AS322L1 and EC225. Its newest product, the 16-passenger EC175 will fly by the end of this year, with certification in 2011 and delivery from and service entry in 2012, first in the oil and gas industry.
Harbin Aviation of China co-develped and will co-build the type, which it designates Z-15.

Avionics derive from those of the EC225 and include a glass cockpit with a four-axis dual duplex autopilot and flight management system in an open architecture. It employs a Spheriflex main rotor and advanced blade profiles and tips on main and tail rotors. It uses Pratt & Whitney PT6C-67AE engines, with full authority digital engine control (FADEC). Main missions comprise oil and gas industry transport and SAR, with others including para-public, security/police, EMS and corporate and VIP transport.

In May, Eurocopter revealed the Stylence executive version of the EC145, joining similar variants of the EC130, EC120, AS350 B2/3 and AS 355 NP.

Sikorsky’s civil types comprise the S-76, S-92 and Schweizer 333.

The S-76D medium twin first flew on 7 February 2009. It features a composite main rotor system with all-composite, flaw-tolerant rotor blades, dual-speed rotor with the Active Vibration Control System (AVCS), Quiet Tail Rotor, optional main/tail rotor Ice Protection System (RIPS), Thales Top Deck avionics, four-axis fully coupled autopilot, EGPWS, traffic collision avoidance system (TCAS), weather radar, health usage monitoring system (HUMS), digital map and Pratt & Whitney Canada 210S turboshaft engines with dual FADEC controls. It could accommodate 13 passengers. Missions consist of offshore oil transport, EMS, executive, SAR and airline. It could receive certification and deliveries begin in 2010.

The FBW version of the S-92, the S-92F first flew in December 2007.

Sikorsky flew the Schweizer 434 light turbine helicopter on 18 December 2008 and plans to certificate it this year. An evolution of the Schweizer 333 it uses a more powerful Rolls-Royce 250-C20W engine.

The five-seat Robinson R66, that company’s first turbine helicopter, will use the new Rolls-Royce RR500 turboshaft.

It should receive certification by 2010.

Bell/Agusta designed the BA609 as a nine-passenger aircraft that cruises at an altitude of 25,000ft, with a range of 750nm. It features a glass cockpit and FBW flight controls and the use of composites in its structure and produces low noise. Bell/Agusta will offer standard, executive and air medical variants. The BA609 first flew in 2003. Testing will eventually encompass four prototypes, with the company envisaging 2012 certification.
 
Conclusion
Innovative rotorcraft configurations, particularly tiltrotors and compound helicopters, offer greater capabilities but at greater cost, so that their best prospects lie in fulfiling niche requirements.

Efforts to improve efficiency and reduce emissions and noise, led by Europe, may produce smaller advances, but ones more readily adaptable to the range of rotorcraft.

Otherwise much rotorcraft development centres on avionics and systems.